Delta Air Lines aircraft at Salt Lake City International Airport on Feb. 16, 2011. / By Ben Mutzabaugh, USA TODAY

by Ben Mutzabaugh, USA TODAY

by Ben Mutzabaugh, USA TODAY

Rivals appear to be selectively matching Southwest's latest fare sale, which includes fares as low as $97 round trip on routes of 500 miles or less.

Southwest rolled out the sale Tuesday, offering distance-based sale prices - which also are offered on subsidiary AirTran. They break down as follows:

$49 each way (500 miles or less)

$99 each way (501 to 1,250 miles)

$129 each way (1,251 to 1,750 miles)

$149 each way (More than 1,750 miles)

A Wednesday morning spot-check of fares shows Southwest's traditional airline rivals matching on many routes, such as Detroit-Baltimore ($98 round trip on Delta), Dallas-Houston ($97 on United) and Los Angeles-San Francisco ($98 on Delta, United and Virgin America).

Still, the matching efforts appear to be selective. Between Chicago and St. Louis, for example, Southwest's $98 round trip sale fare is well below the next-cheapest fares ($204 on American and United) found during our spot check.

On other routes, however, Southwest's rivals appear to have undercut the low-cost giant. Take the Seattle-Boston route, for example. Southwest's sale fare of $328 round-trip is about $80 more than the lowest fares being offered by United ($243) and Delta ($250) as of Wednesday morning.

Southwest's fare sale will come to and end at midnight on Thursday (June 13), and it's likely that rivals' fare-matching will end then as well.

The spot-check of fares was conducted on Kayak.com and Southwest.com about 10 a.m. ET on Wednesday. Fares are for flights departing on Sept. 24 and returning Sept. 26. Scroll down for the details of what we found in our spot check. Unless otherwise noted, all fares are for nonstop flights.